The microwave sky as it looked with 1965 technology // NASA/WMAP Science Team

Two scientists, Arno Penzias and Robert Wilson, discovered the cosmic microwave background in the mid-1960s while testing a large radio antenna for Bell Laboratories. They detected a constant signal no matter what direction they studied. Astrophysicists who had been researching the Big Bang theory of the universe, and possible evidence for it, heard of the engineers’ observations and knew immediately what it was. If the universe had begun in a hot and tiny state, then it would have cooled as it expanded. Eventually, the cosmic temperature would reach just about 3 kelvins above absolute zero, or –270.15° Celsius. This temperature coincides with a microwave radiation glow no matter what direction scientists observed. The discovery of the cosmic microwave background, or CMB, earned Penzias and Wilson the 1978 Nobel Prize in physics.

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